This invention relates to continuous dynode electron multipliers ("CDEMs"). More particularly, it deals with replacing such multipliers, when they wear out.
As described more fully in U.S. Pat. No. 3665497 to Deradorian et al., electron multipliers have been used for years to increase ion, electron, neutral or photon signals. The increase generally ranges from the order of 10.sup.4 to 10.sup.8, depending upon the structure involved.
The Deradorian structure is shown in this application's FIG. 1. It comprises a flared inlet 2 with a stem 4--known collectively in the trade as a funnel. The stem is connected, by electrically conductive adhesive, to a series of spiraled tubes 6. These tubes 6 are made of a lead-glass compound and each tube has an inner channel (not shown) that is coated with a secondary electron emissive surface.
CDEMs have many different configurations. Some have flared inlets, while others do not. To avoid feedback, many are either spiraled or bent, and some are even straight tubes with their inner channels spiraled instead. Nonetheless, each multiplier tube is made of a lead-glass compound like Deradorian's; and each has an inner channel that is coated with a secondary-emissive layer.
Electrical contacts (not shown) are deposited onto Deradorian's inlet 2 and the outlet end 8 of tubes 6. This allows good electrical contact between an external voltage source and the CDEM. This voltage source serves a dual purpose: it charges the secondary-emissive surface, inside the channel; and it draws the electrons through the channel, accelerating them along the way.
Electrons enter Deradorian's flared inlet 2, where they are directed to the tubes 6, by the applied voltage. As they hit the secondary-emissive wall, each electron breaks off a new counterpart, and each pair continues to multiply by factors, typically greater than one, as they travel downstream.
It has been proved that CDEMs produce high gains at low voltage, with little accompanying electrical noise. In addition, they are compact, with this application's FIG. 2 sketches being larger than their real-life counterparts.
Due to these characteristics, CDEMs have achieved widespread use in scientific and medical instruments. In almost all cases, the internal structures of these instruments are quite compact, especially when available space is a limited commodity.
CDEMs work well, but like all parts they eventually wear out. Most CDEMs last about one year. After they are exhausted, electron multipliers usually can be replaced. However, due to the compact nature of the equipment involved, this is often a tedious and delicate task.
Most times, the entire multiplier has to be replaced. However, there are some multipliers that are segmented, with front and rear portions. Such devices are shown in Deradorian's aforementioned patent and U.S. Pat. No. 3312857 to Farnsworth. In both types, the front section is approximately equal in length to the rear section; and the rear section could possibly be replaced once before unsatisfactory gains occur.
Accordingly, it is a primary object of the present invention to provide a specially segmented CDEM, which allows for multiple replacement of its rear section before unsatisfactory gain degradation occurs.
It is another object to provide a segmented CDEM with a removable rear section, wherein the CDEM is extremely simple in design and easy to repair.
It is yet another object to provide a CDEM, commensurate with the above-listed objects, which is highly reliable during use.